The present invention relates to a method of forming a resist pattern by lithography and an anti-reflective layer used in forming the resist pattern.
An anti-reflection technology, to suppress reflection of exposure light from a substrate, has been known as a peripheral technology of lithiography that meets the necessary conditions of dimensional precision and resolution required of ULSI manufacture. When exposure light is reflected from the substrate, a thin-film interference occurs in a resist film, producing exposure variations in the direction of resist fill thickness, called standing waves, and pattern dimension variations called multiple interferences caused by resist film thickness variations. The former degrades the resolution, while the latter deteriorates the dimensional precision. Halation, which is caused by exposure light being reflected on uneven surfaces of the substrate in diagonal directions and in random directions, poses a problem that areas that are originally intended to be shielded from exposure are exposed, making it impossible to form a desired pattern. These problems depend on the intensity of the reflected light from the substrate. The more the reflected light is reduced, the more these problems are mitigated. For this reason, growing efforts are being focused on the reduction of the reflected light from the substrate.
The anti-reflection methods may be classified largely into two groups by their working principle. One group of methods uses as an anti-reflective film a so-called photoabsorptive film with a strong capability to absorb exposure light, and the second group utilizes light interference to prevent reflection. As a representative for the former, an ARC (Anti-Reflective Coating) method is available, which applies a photoabsorptive organic film over the substrate before applying a resist. Light that has passed through the resist toward the substrate is absorbed by this photoabsorptive organic film before being reflected by the substrate surface, so that the light reflected from the substrate and returning to the resist is mitigated.
Examples of anti-reflective films of the second group include Si and TiN. The anti-reflective film of Si, SiOxNy:H,TiN, etc. is deposited over a metal such as W and Al to such a thickness that the reflected light from the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface are in opposite phase with each other in order to reduce the reflection. Conventionally, these methods have been employed in reducing the reflection of light.
The ARC method is described in the Proceedings of SPIE, 1991, vol. 1463, pp. 16-29 and in Japan Patent Laid-Open No. 93448/1984. The anti-reflective film using light interference is described in Japan Patent Laid-Open No. 6540/1984 and 130481/1982, in the Proceedings of SPIE, 1994, vol.2197, pp.722-732 and in the Technical D)igests of International Electron Device Meeting, 1982, pp. 399-402.
Regarding the problems of the conventional anti-reflection technology, explanations are given separately to the anti-reflection technology using light interference and to the ARC technology using light absorption.
The anti-reflection method using light interference requires the reflectivity of the resist/anti-reflective film interface and the reflectivity of the anti-reflective film/substrate interface to be equal in order to cancel the reflected light from these interfaces. Because the reflected light from the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface need to be in opposite phase, the thickness of the anti-reflective film must be made constant at any location. This is close to impossible to realize on uneven surfaces of the substrate because, as shown in FIG. 2, a thickness 21 of a stepped portion of the anti-reflective film is greater than that of a thickness 22 of a planar portion. When the substrate surface layer is a transparent film such as a silicon oxide film, the reflected light from the reflective interface under the silicon oxide film and the reflected light from the resist/anti-reflective film interface must be set in opposite phase. This requires a precise film thickness control including the silicon oxide film. If the silicon oxide film is used as an inter-layer film on the uneven surface of the substrate, the film thickness control is impossible because the thickness of the silicon oxide film varies greatly depending on locations. In these cases, therefore, satisfactory anti-reflection cannot be obtained with the light interference anti-reflection film. There is another problem that, to make the reflectivities equal, it is necessary to optimize the complex index of retraction of the anti-reflective film material according to the substrate material (more precisely, the complex index of refraction of the substrate material). That is, the anti-reflection method using light interference makes it necessary to change the anti-reflective film material each time the substrate material is changed. The anti-reflection method therefore lacks versatility.
Because it deposits a film, the ARC method has the advantages of being simple and versatile, that is, it does not depend on the substrate material. On the other hand, the ARC method has a problem that the anti-reflective film has a large thickness and therefore this method is not suited for forming a microfine pattern. When there is a step in the substrate surface, the thickness 31 (see FIG. 3) of the anti-reflective film 30 over a stepped portion is smaller than the thickness 32 at the side of the stepped portion and the thickness 33 of the planar portion. This makes it necessary to set the thickness over the stepped portion greater than is required. Further, when performing lithography on a planar substrate. the anti-reflective film should be formed thick. To increase photoabsorbance in the anti-reflective film and at the same time reduce the film thickness requires increasing the photoabsorbance level of the anti-reflective film. As the photoabsorbance level increases the reflectivity of the interface between the anti-reflective film and the resist increases, making it impossible to produce a satisfactory reflection prevention effect. For example, when the extinction coefficient of the anti-reflective film, which indicates photoabsorption, exceeds 0.5, the interface reflection between the resist and anti-reflective film much increases. Hence, to obtain a sufficient reflection prevention effect, the thickness of the anti-reflective film needs to be increased. When a thick anti-reflective film is used, the ratio of film thickness to pattern width, i.e., an aspect ratio, becomes extremely large in microfine patterns, making the anti-reflective film very difficult to process. At the same time, the formed pattern will easily collapse, resulting in a faulty product. For example, if a 0.2 xcexcm pattern is to be formed at xc2x15% precision, the reflectivity of the substrate needs to be kept within 0.23% (energy reflectivity). To achieve this reflectivity requires the thickness of the anti-reflective film to be 0.15 xcexcm or greater because of the relationship between the above mentioned photoabsorbance level and the interface reflection. The aspect ratio for the pattern is 0.75. Finer patterns require higher dimensional precision and accordingly the reflectivity must be reduced further, forcing the thickness of the anti-reflective film and the aspect ratio to become still larger.
The present invention has been accomplished with a view to overcoming the above-mentioned problems experienced with conventional technologies.
That is, it is an object of this invention to provide a resist pattern forming method and an anti-reflective film used in the method, which can produce a satisfactory reflection prevention effect even when there are large steps in a substrate surface; which can be used widely irrespective of the substrate material without being affected by reflections from a substrate having a high reflectivity; which can produce a satisfactory reflection prevention effect even when the thickness of the anti-reflective film cannot be made large due to restrictions on the aspect ratio; and which can form microfine patterns with high dimensional precision.
It is a further object of the present invention to provide a resist pattern including an anti-reflective film, particularly useful in the preparation of semiconductor devices, especially in the preparation of microfine circuits of ULSI.
It is a still further object of the present invention to provide a method of etching a substrate (e.g., a layer on a member), especially for etching a substrate having an uneven surface (e.g., a layer on a member having an uneven surface), using a resist pattern including an anti-reflective film, and the product formed.
It is a still further object of the present invention to provide a method of forming a semiconductor device, especially an ULSI, including an etching process, e.g., to form a microfine circuit, the etching process using a resist pattern including an anti-reflective film, and the semiconductor device formed.
To solve the above problems and achieve the above objects, three methods were invented.
A first method is to form on the substrate to be processed an anti-reflective film whose exposure light absorbance is greater on the substrate surface side than on the resist surface side. This method solves the above problems and achieves the above objects.
The methods to change the photoabsorbance for exposure light include the following:
(1) After a film with a high photoabsorbance is formed over the substrate, the surface of the film is exposed to a liquid or gas chemical to diffuse the chemical into the film to decompose the light absorbing component that has reacted with the chemical and thereby to make the light absorbance distributed.
(2) After the film with a high exposure light absorbance is formed over the substrate, a mixing layer of the photoabsorbance film and a resist is generated when applying a resist so that the mixing layer has a variation in the photoabsorbance.
(3) The anti-reflective film is formed by CVD (chemical vapor deposition). During the process of forming this film, the condition of film forming (such as gas composition) is changed to change the photoabsorbance.
(4) The anti-reflective film is formed by sputtering. During the process of forming this film, the composition of ambient gas is changed to change the photoabsorbance.
(5) After the substrate is formed with a film containing a photoabsorptive compound which has a property of being evaporated by heat, the resulting formed film is heat-treated.
(6) The substrate is formed with a film, which has a property of absorbing pattern exposure light and also a property of absorbing light of a certain wavelength (photoabsorption modulation light), reacting with the photoabsorption modulation light and progressively losing the ability to absorb the pattern exposure light. Then, the entire surface of the film is irradiated with the photoabsorption modulation light to form an anti-reflective film whose pattern exposure light absorbance is smaller at the surface of the film than at a deep part of the film.
(7) The substrate is formed with a film, which has a property of absorbing pattern exposure light and also a property of absorbing light of a certain wavelength (photoabsorption modulation light), and, when subjected to heat treatment after being irradiated with the photoabsorption modulation light, progressively losing the ability to absorb the pattern exposure light. Then, the entire surface of the film is irradiated with the photoabsorption modulation light and then baked to form an anti-reflective film whose pattern exposure light absorbance is smaller at the surface of the film than at a deep part of the film.
A second method forms a two-layer reflective film consisting of upper and lower layers over the substrate. The upper layer is an interference film for the exposure light, and the lower layer has a higher exposure light absorbance than the upper layer.
A third method forms a two-layer film consisting of upper and lower layers over the substrate, with the upper layer working as an interference film for the exposure light and the lower layer reflecting the exposure light. The upper layer may be formed as a single-layer film or a multi-layer film.
These methods solve the above-mentioned problems and achieve the above-mentioned objects of the present invention.
The working of the first method is described below.
The reason that increasing the photoabsorbance level of the anti-reflective film does not necessarily result in a decreased reflectivity is that the reflectivity of the interface between the anti-reflective film and the resist increases as the photoabsorbance level of the anti-reflective film rises. Let the complex index of refraction of the anti-reflective film and resist be n1xe2x88x92ik1 and n2xe2x88x92ik2. The intensity of light passing through the anti-reflective film attenuates by exp(xe2x88x924xcfx80k2d/xcex). Reflected light of ((n1xe2x88x92n2)2+(k1xe2x88x92k2)2)/((n1+n2)2+(k1+k2)2) is generated at the resist/substrate interface. As the k1 representing the photoabsorbance level increases, the reflection from the interface between the anti-reflective film and the resist increases. Symbol d represents the film thickness of the resist and xcex represents the wavelength of the exposure light. k1 and k2 are also called extinction coefficients for respective materials.
This invention gradually changes the photoabsorbance of the anti-reflective film beginning with its surface to prevent reflection from the resist/anti-reflective film interface while providing the film with a high photoabsorbance level, thus securing a high anti-reflection effect. That is, by progressively changing the extinction coefficient k starting with the resist and ending with the anti-reflective film, the reflection is attenuated. Although a slight reflection is generated each time k changes, because the reflection surface shifts slightly at each change, the phase of the reflected light changes slightly canceling the beams of reflected light. As a result, the overall reflection decreases. Because the reflection from the interface is reduced for this reason even when the extinction coefficient of the anti-reflective film is large, a high reflection prevention effect can be produced. Since the extinction coefficient of the anti-reflective film can be increased without a restriction of the interface reflection, the use of this anti-reflective film assures a satisfactory reflection prevention even with a substrate having a high reflectivity or with a substrate whose upper layer is transparent. Especially when the extinction coefficient k1 of the anti-reflective film exceeds 0.6, this method has an extreme anti-reflective effect.
The interface reflection prevention utilizes a sort of interference phenomenon. Because what is utilized for the interference phenomenon is only a portion of a certain thickness on the upper surface side of the anti-reflective film, a sufficient anti-reflection effect can be obtained even when the film thickness of the anti-reflective film changes, as long as it is thicker than a certain thickness. Therefore, the reflection prevention is not influenced by stepped portions in the substrate surface.
On the other hand, the conventional anti-reflective film using the interference phenomenon has the reflected light from the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface interfere with each other. Hence, when the film thickness of the entire anti-reflective film changes, a sufficient reflected light attenuation effect cannot be obtained, leaving the reflection prevention to be greatly influenced by the stepped portion on the substrate surface. The portion in the anti-reflective film whose light-absorbance varies should preferably have at least xcex/4nxcex, where N80  represents an average refractive index (real part) of the anti-reflective film within that film thickness and xcex represents the wavelength of the exposure light. It is particularly desirable that the portion has a thickness equal to an odd number times xcex/4n80 . The methods of progressively changing the value of k along the depth direction include one that continuously changes k and one that changes it stepwise little by little. When the diffusion phenomenon and the mixing phenomenon are utilized, the continuously changing method is easier in terms of process.
The methods (6) and (7) described in the foregoing first form over the substrate an anti-reflective film, which has a property of absorbing the pattern exposure light and a certain kind of light (referred to as photoabsorption modulation light), and which, upon reacting with the photoabsorption modulation light, loses the property of absorbing the pattern exposure light. These methods then irradiate the photoabsorption modulation light against the entire surface of the anti-reflective film. The photoabsorption modulation light attenuates in the anti-reflective film according to Lambert-Beyr""s law and its attenuation distribution in the direction of propagation of the photoabsorption modulation light starts from the surface of the anti-reflective film. At the same time, there is generated a photoabsorption distribution of the pattern exposure light beginning with the surface of the anti-reflective film. That is, as indicated by a photoabsorption characteristic curve 51 in FIG. 5xe2x80x94in which the position of the surface of the anti-reflective film is represented by 0 and the film thickness is represented by dxe2x80x94there is produced in the anti-reflective film a photoabsorption distribution, in which the photoabsorption of the pattern exposure light is weak at position 0 on the surface of the film and then progressively increases in the direction of depth. This distribution starts with the surface of the film and thus does not change even when the thickness of the anti-reflective film varies according to locations due to the stepped portions in the substrate as shown in FIG. 3. That is, the pattern exposure light absorption distribution for the thin film area at the top of the stepped portion (34 of FIG. 3) and that for the thick film area at the bottom of the stepped portion (35 of FIG. 3) are hardly different from each other, as shown in FIG. 6. The photoabsorption distributions are equal up to the depth do to which the photoabsorption modulation light penetrates, and in deeper areas they exhibit constant photoabsorptions. Symbols d1 and d2 in FIG. 6 represent the positions on the substrate for the levels 34 and 35, with the surface of the anti-reflective film taken as 0, and have the same values as the film thicknesses 31 and 32. To describe more precisely, it is not in the direction of film thickness but in the direction of light propagation that the photoabsorption distribution does not change. When, for example, the surface of the anti-reflective film has a slope caused by the stepped portions of the substrate, the photoabsorption modulation light 41, as shown in FIG. 4, bends at the surface of the anti-reflective film. In this bent direction the same photoabsorption distribution can be obtained. In FIG. 4, reference numeral 42 represents a substrate with a stepped portion; 43 a portion of the anti-reflective film in which the absorption level (extinction coefficient) changes (i.e., there is a gradient in photoabsorption); and 44 a portion of the anti-reflective film that has a constant photoabsorption level. Because the pattern exposure light also bends at the interface of the anti-reflective film, a uniform photoabsorption distribution for the pattern exposure light can be obtained, thus reducing reflection without being influenced by stepped portions.
With this process, the extinction coefficient k can be varied gradually from the resist to the anti-reflective film, thereby reducing the reflection that is caused by rapid change of k. Although a slight reflection is generated each time k changes, because the reflection surface is shifted progressively, the phase of the reflected light changes little by little, producing a canceling effect. Thus, the reflection as a whole decreases. Because the reflection from an interface is reduced in this way, a high reflection prevention effect can be obtained even if the extinction coefficient of the anti-reflective film is large. Since the extinction coefficient of the anti-reflective film can be increased without a restriction of the interface reflection, the use of this anti-reflective film ensures a satisfactory reflection prevention either with a substrate having a high reflectivity or with a substrate whose upper layer is transparent. The interface reflection prevention utilizes a sort of interference phenomenon. Because what is utilized for the interference phenomenon is only a portion of a certain thickness on the upper surface side of the anti-reflective film, a sufficient anti-reflection effect can be obtained even when the thickness of the anti-reflective film changes, as long as it is thicker than the certain thickness. Further, because the thickness used for the interference effect is well controlled in the direction of light travel, the anti-reflection rate is high. Therefore, the reflection prevention is not influenced by the stepped portions of the substrate. On the other hand, the conventional anti-reflective film using the interference phenomenon has the reflected light from the resist/anti-reflective film interface and the reflected light from the anti-reflective film/substrate interface interfere with each other Hence, when the film thickness of the entire anti-reflective film changes, a sufficient reflection attenuation effect cannot be obtained, leaving the reflection prevention to be greatly influenced by the stepped portion on the substrate surface. The portion in the anti-reflective film whose light absorbance varies (43 in FIG. 4 and O-do, position range in FIG. 6) should preferably be at least xcex/4n80 , where N80  represents an average refractive index (real part) of the anti-reflective film within that film thickness and xcex represents the wavelength of the exposure light. It is particularly desirable that the portion has a thickness equal to an odd number times xcex/4n80 .
The anti-reflective film, which has the property of absorbing the pattern exposure light and, upon reaction with the photoabsorption modulation light, loses the property of absorbing the pattern exposure light, can be produced by diffusing in an organic film a photoabsorptive compound that becomes transparent as it is exposed to light, a so-called bleaching characteristic. Alternatively, it may be obtained by using a film, which has a property of absorbing photoabsorption modulation light, and, when subjected to heat treatment or chemical treatment after being irradiated with the photoabsorption modulation light, losing the ability to absorb the pattern exposure light.
An illustrative (but not limiting) group of compounds having this bleaching characteristic is the nitrone compounds. These compounds are described in Hamer, et al., Chem. Rev., 64, 472 (1964) at 474, the contents of which are incorporated herein by reference in their entirety, and are compounds containing the group: 
A specific type of nitrone which can be used in the present invention is set forth in the following: 
When this specific type of nitrone is irradiated with light (hv), it reacts to form the following compound (bleached): 
The working of the second method is described below.
The lower layer has a high absorbance of exposure light and shields light reflected from the substrate. The lower layer therefore offers a satisfactory reflection prevention effect even with a substrate with high reflectivity and a substrate having a transparent film. Because the shielding of reflected light from this substrate uses a photoabsorption effect, not interference, it does not depend on the material of the substrate. Generally, however, the use of a photoabsorptive film that is thin and able to shield well the reflected light from the substrate results in an increased imaginary part of the refractive index (extinction coefficient) of the anti-reflective film and an increased reflectivity at the resist/anti-reflective film interface, making it impossible to produce a good reflection prevention effect. This method solves this problem by using the interference film of the upper layer. That is, the reflected light from the resist/upper layer anti-reflective film interface and the reflected light from the upper layer anti-reflective film/lower layer anti-reflective film interface with each other in such a way that these reflected lights cancel each other (the upper layer film thickness is set so that the phases of the reflected lights are opposite to each other). The combined effect of the interface reflection reduction by the upper layer anti-reflective film and the substrate reflection reduction by the lower layer anti-reflective film solves the above problem.
The working of the third method is explained below.
The anti-reflective film of the third method consists of an upper layer film and a lower layer film. By reflecting the exposure light by the lower layer film, phase and intensity control of the reflected light can be achieved. That is, because the reflecting surface is not the substrate surface but the lower layer film surface, the reflected light has a constant phase and intensity regardless of optical constants of the substrate and whether there is a transparent film or not. The reflected light is cut by the tipper layer""s interference film. In other words, the reflected light from the resist/upper layer anti-reflective film interface and the reflected light from the upper layer anti-reflective film/lower layer anti-reflective film interface are made to interfere with each other so that they cancel each other (the upper layer film thickness is set so that the phases of the reflected lights are opposite). The reason that the reflected light can be cut by the upper layer""s interference film is that the phase and intensity of the reflected light can be kept constant. This is achieved by the introduction of the reflective film. The fact that a reflective film is introduced in realizing the reflection prevention is a feature of this aspect of the present invention.